public class aka20165211 {
    public static void main(String[] args) {
        int [] tmp=new int[args.length];
        for (int i=0;i<args.length;i++){
            tmp[i]=Integer.parseInt(args[i]);
        }
        if(tmp[0]<tmp[1]){
            System.out.println("error!");
        }
        else{
            System.out.println(C(tmp[0],tmp[1]));
        }
    }
    private static int C(int n,int m){
        if(n==1)
            return m;
        else
            return C(n-1,m-1)+C(n-1,m);
    }
}
